Just for the record, some details of John Margas, the son of Jacob Margas:
John Margas was recorded as an Optician, he was apprenticed to Nathaniel Adams in 1735 and turned over to John Cox in 1741/2. Free of the Spectaclemakers' Guild 1742, his working period is thought to be 1743-1767.
He was noted at the following addresses:
1745 - Charing Cross, London
1745 - Golden Spectacles in Long Acre near James Street, London
1753-1758 - Rose Street, near Long Acre, London
1759 - Dublin
1761-1767 - Capel Street, Dublin
Details of the known apprentices of John Margas can be found here: viewtopic.php?f=38&t=8565&p=76732#p76732
John Margas was bankrupt in 1758, perhaps this event prompted the move to Dublin. He died late 1767.